nice find on the valve specs, its nice to realise how much simpler all of this is with the internet, I remember transplanting a YZ250 engine into a later model rolling chassis a very long time ago, took me months and months to get the carb to cylinder rubber to suit, plus a few needless purchases in the process.

It will be interesting to see if the valve head diameter dimension changes larger inlet/smaller exhaust and vice versa are to safeguard against potential valve strike between valves at high revs during the overlap period - speculation.

Anyway some really interesting news from L.A. Sleeve, I enquired about 4 stock sleeve dimensions in particular, 52mm, 53mm, 54mm and 55mm bore, conversion to metric is my addition.
The response from L.A. Sleeve is below the links to the products I enquired about

I'm ordering a CBR400RR STD (55mm) piston from aliexpress to have a close look at it

Their response - check the minimum wall thickness they say is possible :O

On the smaller bore aluminum cylinders, we recommend .004” interference pressed fit. Normal install procedures is to heat the cylinder and chill the sleeves. They’ll drop right into place. As the cylinder cools on top of the sleeve, we’ll put them under a hydraulic press to keep it from creeping up as it cools. Minimum wall thickness on the smaller bore sleeves we recommend holding is 1.00mm. The thicker the better though.

We can custom build any sleeves you may need. But, hopefully one of our stock sleeves will work for your project.

The L.A. Sleeve FL-52 actually ticks all the boxes, boring to 55mm would leave 1.67mm wall thickness and it also leave a small fraction more material in the aluminium barrel than @gregt did, I think we're in a safe zone, with the potential for 0.25mm and 0.5mm oversizes

Depends on the material. Did LA Sleeve say what the material is that they use ?
1mm is possible as a liner inside an existing bore. Not what you want here.
I'd class the FZR sleeves as "not well supported"

I'll have a measure tomorrow to see how big the valve seats are in the head - that's your limiting factor..
Given how narrow the valve angles are - and how far apart front to rear - I don't think valve to valve clearance is a problem.

L.A.SLEEVE manufactures our cylinder sleeves from a centrifugally spun cast iron alloy of carbon-chrome and molybdenum. The MOLY 2000 iron alloy material features many of the same alloys that are used in todays plated aluminum cylinders. Unlike plated aluminum bores, sleeves are boreable.

These centrifugally spun cast sleeves feature a high tensile stength of 48,000 psi to 53,000 psi which insures ease of installation, trouble free boring or machining.

Inlet valve seat OD is 20.0mm Exhaust is 17.5mm OD Both measured directly off the recess less insert..That happens when you bandsaw through the inserts.
You can usually take the seat OD up to the edge of the insert with some careful cutting so around 19mm inlet valves and 17mm exhausts should be possible. The throat of course would be a taper cut.
You'd have to do some serious searching though for a head shop with the rare teenie tiny Serdi cutter set....
They're pretty thin on the ground over here but i do know of a couple who have the smallest one available.

Seems like 2KR exhaust valves @ 16.5 are the largest from the FZR series (4mm stem diameter though) and 3LN Intakes @ 19mm are potentially good to go if yours are 18mm intake to begin with.
Speculation and pondering: I wonder if later FZR's and the Zeal used smaller intake valves as part of the detuning process in combination with smaller carbs and altered spark advance and a different cam profile/timing.

No idea what version my head is from. One head and two bottom ends - all loose. Only ID I can see on the head is on the back of the camchain tunnel. No way of telling even if that head was on one of the bottom ends...
This is the casting ID marks.
1HX
01
Y-3

Valve sizes are inlet 18mm head dia, Ex 15.5mm head dia.
Just looked again at your ebay sizes ex china - odd mix.
Can their quoted sizes be relied on ?

Looks like 3LN intake @ 19mm and 2KR exhaust 16.5mm will get you a size increase on both.

My guess is that you may have a zeal head with your valve sizes, though I can't find those specs published anywhere to be certain

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I'd seriously doubt I've got a Zeal. The bits came from the wild West coast of the South Island - and i understand they've been apart for at least 10 years...My pick is that Yamaha juggled valve sizes between EXUP and non EXUP models.
The EXUP IMO would probably have had the bigger exhaust valves as they could sacrifice some gas speed once the exhaust throttling valve was in the system. Without it, smaller valves would have given more midrange.

@gregt it's difficult to make heads or tails of the different valve sizes as yours are both the smaller sizes listed, this is assuming that any of the figures we have are actually correct, though the figure of ~3.5mm stem diameter says it's definitely not a 2KR. This assumes the ebay auction dimensions are correct also.
3LN6/7 from this page lists lower peak output figures and higher peak torque at lower RPM- http://my-x15.net/FZR_INFO.html
yours may actually be a 3LN6/7 cylinder head, not that it's of much actual importance.

@Linkin - the engine you're got disassembled already, 3LN5 correct? What are the valve head diameters?

@gregt it's difficult to make heads or tails of the different valve sizes as yours are both the smaller sizes listed, this is assuming that any of the figures we have are actually correct, though the figure of ~3.5mm stem diameter says it's definitely not a 2KR. This assumes the ebay auction dimensions are correct also.
3LN6/7 from this page lists lower peak output figures and higher peak torque at lower RPM- http://my-x15.net/FZR_INFO.html
yours may actually be a 3LN6/7 cylinder head, not that it's of much actual importance.

@Linkin - the engine you're got disassembled already, 3LN5 correct? What are the valve head diameters?

Click to expand...

I have 1989 2KR with EXUP apart that belongs to risky, but only a 3LN7 bottom end. The heads and barrels are at the fizzer factory along with all the other spare engines.